1. Field of the Invention
The present invention relates to a thermal transfer ink layer composition, wherein polyvinylacetal resin and cellulose derivatives are used in a mixture for the binder in the thermal transfer ink layer composition for the dye-donor element used in sublimation thermal dye transfer, or another type of a binder is mixed into the mixture of polyvinylacetal resin and cellulose derivatives and so used therein. As a result, the adhesion between the ink layer and substrate film, and the gradation property of printed images are improved as compared to the case in which the respective binders are used individually. More particularly, the present invention relates to the thermal transfer ink layer composition of the sublimation thermal dye transfer ink ribbon, which can produce high quality images by means of the aforementioned improvements.
2. Description of the Prior Art
The electrophotography, ink jet, thermal transfer methods, etc are generally used for printing color images. Among these methods, the thermal transfer methods have advantage in that they produce no noise during printing, the apparatus therein is relatively inexpensive and compact, and maintenance thereof is easy. Depending on the physical property of the transfer ink layer, the thermal transfer method can be further categorized into the melting and sublimation type methods.
Under the melting type method, the transfer ink layer is heated and melted by means of the heating element, after which is transferred to the receiving element and solidified. During the transfer process, not only the colorants but also the binder are transferred to the receiving element. As such, in the above method, it is impossible to obtain an image having an essentially continuous gradation as in photograph. On the other hand, under the sublimation type method, the thermal transfer ink layer consists of the sublimable dye having heat transferability and the binder resin. Only the dye in proportion to the applied heat by means of the heating element is transferred to the receiving element, and the images are formed on the receiving element. As such, the method is commonly called the dye diffusion thermal transfer type. Under the sublimation type method, the amount of the transferred dye is controlled by the applied thermal energy. Consequently, the method has an advantage in that it is possible to obtain an image having continuous gradation. As such, the method is commonly used in the automated office equipment, such as facsimile and copier in addition to the areas in CAD, CAM, and graphic design. The method is also widely used in an area in which the electronic images from color video camera are printed via color printer.
The recording principle of the sublimation thermal transfer printing method is as follows: Under said thermal transfer recording method, an electronic picture is first subjected to color separation by color filter. The respective color-separated images are then converted into corresponding electrical signals. These signals are then operated on to produce yellow, magenta, cyan and/or black electrical signals. These signals are then transmitted to a thermal printer. To obtain a print, yellow, magenta, cyan and/or black dye-donor elements are placed face-to-face with the dye-receiving element. The dye-donor element and dye-receiving element are then inserted between the thermal printing head and platen roller. The thermal printing head applies heat from the back of the dye-donor element. The thermal printing head has many heating elements and is heated sequentially in response to the yellow, magenta, cyan or/and black signals.
As for the sublimation thermal transfer ink ribbon, one side of the substrate film is applied with the thermal transfer ink layer. Between said ink layer and substrate film, an adhesive layer is formed for the purposes of improving the adhesion between the ink layer and the substrate film. The opposite side of the thermal transfer ink layer may be coated with a heat resistant lubrication layer in order to prevent the printing head from sticking to the dye-donor element.
Further, said thermal transfer ink layer consists of a polymeric binder, sublimable dye which is transferred to the receiving element by means of heat, and other additives. As for preparing the sublimation thermal transfer ink ribbon, the selection of the binder resin with appropriate physical property is of great importance. The requirement of the physical property of the binder resin is as follows:
1. In order to obtain high transfer sensitivity, the dye from the transfer ink layer to the receiving element must be easily transferable. PA1 2. The adhesion must be superior with respect to the substrate of the sublimation thermal transfer ink ribbon. PA1 3. There must be no melt adhesion onto the receiving element during printing. PA1 4. There must be adequate affinity with respect to the dye, and re-crystallization of the dye must not occurred PA1 5. In order to obtain superior gradation, the transfer sensitivity should be low at low printing energy.
As for the results of research for obtaining a binder with the satisfactory requirements as above, the methods for using a variety of binders on the thermal transfer ink layer of the sublimation thermal transfer ink ribbon have been proposed.
U.S. Pat. No. 4,700,207 discloses the use of resins of the cellulose derivatives as binder, namely, cellulose acetate butyrate, ethyl cellulose, cellulose acetate phthalate, etc. However, the resins of said cellulose derivatives lack adhesion with respect to the widely used substrate film, the polyethyleneterephthalate film. For this reason, the transfer ink layer may separate from the substrate film during printing.
Further, the methods of using the resins in the polyvinylacetal, such as polyvinylbutyral (U.S. Pat. No. 4,650,494), polyvinylacetoacetal (European Pat. No. 0,271,861), have been respectively disclosed. A generally high transfer sensitivity can be obtained from polyvinylbutyral, but the high transfer sensitivity at low printing energy results in a poor gradation. As such, polyvinylacetoacetal resin has the disadvantage of having a low transfer sensitivity.